Such continuous driers are used in printing lines where a web is unwound from a roll, passes a plurality of printing units which apply patterns to it in due register, and runs through a thermally operating continuous drier before reaching a folding device where it is processed into the finished copies.
The basic features of continuous driers and their operation are set out in the "Safety Regulations for Printing and Paper Processing Machines" published by Carl Heymann Verlag KG, Order No. ZH/19, Oct. 1984. According to this publication, solvents or substances containing solvents which are introduced into a continuous drier may therein react with air to produce a potentially explosive mixture of solvent vapor and air. As a result of the installed unprotected heating systems, which heat the fresh and circulated air destined for the drying tunnel to a certain temperature, there is no gas-tight separation between the drying tunnel and the burner chamber. The manufacturer or operating company is required to specify a threshold temperature which is lower than the ignition temperature (refer to DIN 51794) and lower than the inflammation temperature at which combustion phenomena will result. According to the definition of unprotected heating systems, the temperature of the heated air flow will partially exceed the threshold temperature. Said threshold temperature has been fixed at 80% of the ignition temperature laid down in DIN 51794.
The inflammable substances are assigned a lower explosion concentration limit at which, after ignition, a flame independent of the ignition source will just cease to be able to propagate on its own. In the case of continuous driers with unprotected heating in the circulated air channel, it is assumed that the solvent vapor concentration immediately upstream of the unprotected heating system will not exceed 25% of the LEL threshold. In the event of an unknown solvent vapor concentration, the LEL of the solvent is assumed to be 20 g/m.sup.3 for reference purposes.
As there exists a threshold temperature which must not be exceeded during continuous drier operation, the heating system for the continuous drier must be equipped with a temperature control system or at least with a temperature restricting device. Applicable rules and regulations also prescribe that the restriction of the solvent vapor concentration in the continuous drier should be monitored by a gas warning system which must reliably register any hazardous quantities of such vapors that might constitute a potentially explosive atmosphere. The sensor of this gas warning system should provide at least five measurements per minute for each measuring point.
The problem exists that these officially prescribed gas warning systems merely provide measured values at given intervals and from a so-called representative point, so that sudden changes in concentration levels or changes in the composition of the vaporizing materials applied to the web cannot be adequately picked up. In addition, the control of the drier process is not based on the actual gas or vapor concentration but on a temperature program for the heating system, which, in the case of unprotected heaters, is derived from ignition flash points.
To monitor the solvent vapors in the continuous drier it is standard practice to install a flame-ionization detector (FID). German Patent No. 2150259 B2 discloses a gas-measuring system attached to an extraction funnel on a low-pressure drier with which the gas proportion of toluene in the air can be determined.
The design of known infrared measuring systems may vary greatly, depending on the individual application. The device designed in accordance with European Patent No. 0123458 has a small test chamber which can be filled with the gas to be measured. Filters are installed to generate radiation in the absorption wavelength area. The invention presented in German Publication No. 211835 operates with interference filters for the absorption bands of the gas constituents to be analyzed. By using laser-induced radiation in a system which, through its very nature, is spectrally restricted to one absorption range, this design is only useful for a single constituent; at least it will require sophisticated equipment to provide tuning facilities for application throughout a wider radiation spectrum. An absorption system using a laser beam is described in U.S. Pat. No. 4,475,816.
Inventions using measuring chambers and sampling systems have the disadvantage that the gas constituent measurement will not yield representative values and is also too slow. FID systems provide delayed indications. Known infrared measuring devices are based on standard components and physical processes which allow the user to resolve a large range of measurement tasks; however, the infrared measuring instruments known for industrial service are based on the inadequate measuring probe principle, while other known infrared measuring devices meet laboratory requirements only.
The problem which arises in the case of the standard continuous drier and its operation, therefore, is to control the drying process in accordance with the concentrations of evaporating matter and in conjunction with the burner control system. Furthermore, the determination of these concentration values should be characterized by a quick response and must be equally suitable for each of the volatile constituents encountered.